It is known that polyvinyl chloride is one of the most important plastics and because of its mechanical properties, its chemical resistance and its flame-resistant properties, it is employed in a great variety of forms as rigid PVC or plasticized PVC. In contrast to most other industrially utilizable thermoplastics its second-order transition temperature (glass transition temperature) is about 80.degree. C., so that the heat distortion point of rigid PVC, which when measured by the Vicat method is between 75.degree. C. and 84.degree. C. depending on the K-value and the formulation, is inadequate for many requirements and a whole range of applications remains closed to PVC. There has therefore been no lack of attempts to achieve an improvement in the heat distortion point by changing the molecular structure by copolymerization, by chemical after-treatment or by admixture of thermoplastics having a higher glass transition temperature than PVC. (G. Kuhne et al., Kunststoffe, Volume 63, (1973), pages 139-142). In particular, post-chlorination and cross-linking and copolymerization with maleimides, have given products which are technically improved and in part find limited uses in practice. The Vicat temperatures achievable by these methods (Vicat B, DIN 53 460) are about 90.degree. C.